Measurement of $Z$ Boson Production with the ATLAS Experiment at the LHC

A measurement of the differential pp → Z/γ∗ → e+e− cross section as a function of di- electron invariant mass in the region 66 < m_Z < 116 GeV using 4.58 fb−1 of data collected at a center-of-mass energy of √s = 7 TeV in 2011 with the ATLAS detector at the LHC is performed. The measurement is performed with a comprehensive treatment of systematic and statistical uncertainties evaluated using Monte Carlo simulation and data-driven techniques. The contribution from electroweak and ttbar background processes is estimated using simulation, while the contribution from QCD backgrounds is estimated using a data-driven method. The measured cross section is compared with various theoretical predictions from parton distribution function (PDF) fits to next-to-next-to-leading order QCD calculations with next-to-leading order electroweak corrections. The precision of the measurement is of the order of 1 − 2% and provides strong PDF discrimination, with none of the state of the art PDFs showing good agreement with the data. The measurement can be used as an input to constrain future PDF fits. A combination of separate electron and muon W± and Z/γ∗ cross section measurements is also presented, with full treatment of each channel's systematic uncertainties and the correlations between them. These measurements are performed as a function of lepton pseudo-rapidity and Z boson rapidity forthe W± and Z/γ∗ production processes respectively. These serve as a precise probe of QCD dynamics and proton PDFs. The combined cross sections are compared with theoretical predictions and strongly discriminate between, and in some cases exclude, different PDF fits. The precision of the measurements is better than the PDF uncertainties, and NNLO QCD and NLO EW theory are limiting factors in interpretation of the data